In recent years, a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of approximately several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs and electro-optical devices, and thin film transistors that are used as switching elements in image display devices are, in particular, urgently developed. There exists a wide variety of metal oxides and such metal oxides are used for various applications. Indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. Examples of the metal oxides having semiconductor characteristics are tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. Thin film transistors in which a channel formation region is formed using such a metal oxide having semiconductor characteristics are already known (Patent Document 1 and Patent Document 2).